The development of various devices and systems in recent years has increased the demand for higher-performance batteries (primary batteries, secondary batteries, capacitors, etc.) as a power source. For example, lithium secondary batteries are gaining widespread popularity as high energy density secondary batteries serving as the power source for electronic devices such as portable communication devices, laptop computers, etc. Further, in terms of reducing environmental load, lithium secondary batteries are also expected to be used as batteries for driving the motors for vehicles. Accordingly, there is a demand for the development of high energy density lithium secondary batteries that will correspond to higher performance in the above devices. In order to meet this demand, increasing the capacities of both positive electrodes and negative electrodes is necessary.
However, the capacity of the positive electrode for currently available lithium secondary batteries has not increased as much as that of the negative electrode. For example, the specific capacity of lithium nickel oxide-based materials, which is said to be relatively high, is about 190 to 220 mAh/g. Even in the case of Li2MnO3-based materials, which contain a comparatively larger amount of lithium per formula weight, their theoretical capacity, based on the assumption that all of the lithium ions are used during charge and discharge, is merely about 460 mAh/g.
In contrast, although sulfur is a substance with a low operating voltage, the theoretical capacity thereof is as high as about 1,670 mAh/g. However, elemental sulfur has low electrical conductivity, and when it is used in a battery system that uses a currently available organic electrolyte (for example, an electrolyte obtained by dissolving 1 M of LiPF6 in a mixed solution of ethylene carbonate and dimethyl carbonate (ethylene carbonate:dimethyl carbonate=1:1)), the sulfur reacts with lithium ions during the discharge of the battery system, and thereby dissolves into the electrolyte. Using metal sulfides (MSx; M represents a metal component such as nickel, iron or copper), which have an electrical conductivity comparable to or higher than that of a semiconductor material and show relatively low dissolution into the electrolyte compared to sulfur, is one approach to overcoming these problems. A metal sulfide has a theoretical capacity of about 600 to 900 mAh/g; although this is lower than that of elemental sulfur, a higher capacity than the currently available oxide material is expected to be achieved.
However, positive electrodes of metal sulfides have a problem in regard to cycle performances. Therefore, in order to promote the use of metal sulfide as a positive electrode material, suppression of its cycle degradation is required. The main probable causes of cycle degradation are: (1) dissolution of sulfur components into the organic electrolyte, (2) electrochemical irreversibility of active materials, (3) separation of active materials from a current collector; etc. In terms of (1), in order to effectively suppress the dissolution of sulfur components, approaches from the standpoints of both electrolytes and active materials may be necessary. Some research from the standpoint of electrolytes has been conducted on organic electrolytes that can suppress the dissolution of sulfur components; however, approaches from the standpoint of the active materials, such as modifying the metal sulfide, have been almost entirely unreported.
There have been some reports (Non-Patent Documents 1-4 listed below) regarding all-solid-state lithium secondary batteries, which use sulfur-based solid electrolyte, wherein, in order to suppress the degradation caused by the oxide-based positive electrode active materials (e.g., LiCoO2, LiNi0.5Mn1.5O4) being directly in contact with the solid electrolyte, the surfaces of the active materials are covered with lithium phosphorate, oxide-based lithium ion conductive materials (Li1.5Al0.5Ge1.5(PO4)3), zirconium oxide, etc. However, in batteries that use an organic electrolyte, an effective means for preventing the degradation of metal sulfide that is used as a positive electrode active material, in particular, means for suppressing the dissolution thereof into an organic electrolyte, has not been yet found.    Non-Patent Document 1: Y. Kobayashi, H. Miyashiro, K. Takei, H. Shigemura, M. Tabuchi, H. Kageyama, and T. Iwahori, J. Electrochem. Soc., 150, A1577 (2003).    Non-Patent Document 2: Y. Kobayashi, S. Seki, M. Tabuchi, H. Miyashiro, Y. Mita, and T. Iwahori, J. Electrochem. Soc., 152, A1985 (2005).    Non-Patent Document 3: Y. Kobayashi, S. Seki, A. Yamanaka, H. Miyashiro, Y. Mita, and T. Iwahori, J. Power Sources, 146, 719 (2005).    Non-Patent Document 4: H. Miyashiro, A. Yamanaka, M. Tabuchi, S. Seki, M. Nakayama, Y. Ohno, Y. Kobayashi, Y. Mita, A. Usami, and M. Wakihara, J. Electrochem. Soc., 153, A348 (2006).